Numerous enhanced oil recovery (EOR) processes and systems have been described and used. Though EOR processes have applications in increasing the recovery of all oils or hydrocarbons from oil-bearing formations they have particular applications in increasing the recovery of the more viscous oils from oil-bearing formations. In general the higher the viscosity of oils in their natural state in an oil-bearing formation, the more difficult it is to produce the oils from the formation. Thus in general a greater amount of high viscosity oil is left in an oil-bearing formation after primary production than is the case in formations having lower viscosity oil therein. Therefore EOR processes find their greatest applications in high viscosity oil-bearing formations.
EOR processes employ displacing fluids which include steam, carbon dioxide, and solvents. EOR processes also employ various combinations of wells including single wells and multiple wells in various patterns.
In U.S. Pat. No. 4,617,996 there is described a method of recovering oil from an oil-bearing formation by injecting a displacing fluid into the formation through an injection well and recovering fluids from a production well which is at a horizontal distance from the injection well. The displacing fluid used is one which is immiscible or near-innniscible with the oil at the oil-bearing reservoir conditions. A mixture of carbon dioxide and a light hydrocarbon such as ethane is described as a preferred displacing fluid. U.S. Pat. No. 4,678,036 describes a miscible flooding process for oil recovery which employs a multiple contact miscible solvent such as carbon dioxide and an additive such as butane which lowers the minimum miscibility pressure of the oil/solvent system. U.S. Pat. No. 4,756,369 describes a process for recovering viscous heavy oil from a substantially shallow reservoir wherein liquid carbon dioxide, a spacer of a vaporizable drive fluid, followed by steam is used.
U.S. Pat. No. 4,844,158 describes a method for stimulating the recovery of viscous oil from a formation penetrated by at least one horizontal wellbore. The wellbore is filled with a solvent such as xylene, toluene, diesel oil, kerosene or mixtures thereof and is displaced from the wellbore upward into the formation by a second fluid having a density greater than the solvent. The solvent mixes with the viscous oil in the formation to reduce the viscosity of the viscous oil. Thereafter injection of the second fluid is stopped and the fluids and oil of reduced viscosity are allowed to flow by gravity into the horizontal wellbore. Suitable second fluids include sea water, brackish water, and aqueous solutions of potassium chloride, sodium chloride, calcium chloride, zinc bromide, potassium bromide, sodium bromide, calcium bromide, and mixtures thereof.